Soil profile
As soil forming processes continue; soil is found to
be arranged in a definite sequence of soil layers or horizons. This vertical
arrangement of various soil horizons is called soil profile.
The thickness of layers varies from place to place.
In places where soil forming processes have been in action for long and if
erosion has no taken place, the layers are thicker making the profile deeper.
In some places where erosion has occurred, the top layers of the profile may be
missing.
Soil profile
Properties of the various
horizons in a soil profile
Each horizon has certain properties that can be used
to distinguish it from other layers. These include, compactness, organic matter
content, presence of living organisms, amount of air and water, soil structure
as well as soil texture. The following are the characteristics exhibited by
each horizons/ layers.
Horizon O (organic horizon)
This horizon contains organic materials that are
different stages of decomposition on the surface of the soil.
It is commonly found in highly vegetated area such
as forests. This horizon is not soil as yet.
Horizon A (Top soil)
In this horizon, there is abundance of living
organisms that are actively breaking down organic materials to organic matter
and humus.
It has plenty of organic which makes it look dark or
black in colour.
Also, because of the presence of organic matter, it
is rich in plant nutrients.
The soil particles are loosely packed which enable
water and air to move freely through it.
Crumb and granular structures are commonly found in
this horizon.
As rain water passes through this horizons, it
carries away plant nutrients to the underlying horizons. This is called
eluviation. It is a layer that is highly
leached.
Horizon B (sub soil).
This is the horizon where most of the materials
leached from horizon A are deposited and is referred to as the zone of
accumulation or illuviation.
Fewer living organisms exist in this horizon.
Organic matter content is low or is lacking in some
places which makes the soil brown or red in colour.
Soil particles are tightly packed which reduces the
amount of air present.
The compactness and the high clay content of soil in
this horizon enables it to hold a lot of water and become water logged.
Block and prismatic structure are commonly found in
this horizon.
Horizon C (partly weathered rock
of parent material)
This horizon is made up of coarse partly weathered parent
material that originates from the parent rock as a result of weathering.
Soil particles are very tightly packed and therefore
poorly aerated.
This horizon lacks organic matter.
Apart from the tree roots, there are no living
organisms in this horizon. It is a layer outside the biological activities.
Horizon D (Bed rock)
This is the original parent material which is still
intact. It has not experienced weathering forces due to protection by the
overlying horizons.
Since it is a solids mass of rock, it lacks living
organisms, organic matter and air.
Importance of soil profile to
crop production
- It can be used to determine the
suitability of soil for agriculture. The deeper the profile especially the
top soil layer, the better the soil profile for crop production. The
deeper soil profile provides the roots of crops with a bigger volume of
soil to exploit.
- It helps to determine the type of crops to
grow. Shallow rooted crops are grown in shallow profiles and vice versa.
- A loosely packed sub soil reduces the degree of
water logging in the top soil layer thereby favouring growth of most
plants.
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